Dr.
Julianne I. Moses
Recent
Research

Mercury's
Polar Deposits

Ground-based
radar observations of Mercury have revealed the presence of unusually
bright radar-reflective regions at the planet's north and south
poles (Slade et al. 1992, Science258, p. 635, and
Harmon and Slade 1992, Science258, p. 640). The similarity
of both the strength and polarization behavior of these radar echoes
to the observed radar characteristics of the icy Gallilean satellites
and the residual south polar cap of Mars prompted the observers
to conclude that water ice or some other volatile material might
be present at the poles of the otherwise hot planet (see also Butler
et al. 1993, J. Geophys. Res.98, p. 15,003).
Although thermal models indicate that water ice can be stable in
permanently shaded regions near Mercury's poles, the ultimate source
of the water remains unclear. Katherine Rawlins, Kevin Zahnle, Luke
Dones, and I have used stochastic models and other theoretical methods
to investigate the role of external sources in supplying Mercury
with the requisite amount of water. By extrapolating the current
terrestrial influx rate to that at Mercury, we find that continual
micrometeoritic bombardment of Mercury over the last 3.5 billion
years could have resulted in the delivery of (360) ×
1016 g of water ice to the permanently shaded regions
at Mercury's poles (equivalent to an average ice thickness of 0.820
m). Erosion by micrometeorite impact on exposed ice deposits could
reduce the above value by about a half. For comparison, the current
ice deposits on Mercury are believed to be somewhere between ~2
and 20 m thick. Using a Monte Carlo model to simulate the impact
history of Mercury, we find that asteroids and comets can also deliver
an amount of water consistent with the observations. Impacts from
Jupiter-family comets over the last 3.5 billion years can supply
(0.1200) × 1016 g of water to Mercury's polar
regions (corresponding to ice deposits 0.0560 m thick), Halley-type
comets can supply (0.220) × 1016 g of water
to the poles (0.077 m of ice), and asteroids can provide (0.420)
× 1016 g of water to the poles (0.18 m of
ice). Although all these sources are nominally sufficient to explain
the estimated amount of ice currently at Mercury's poles, impacts
by a few large comets and/or asteroids seem to provide the best
explanation for both the amount and cleanliness of the ice deposits
on Mercury. Despite their low population estimates in the inner
solar system, Jupiter-family comets are particularly promising candidates
for delivering water to Mercury because they have a larger volatile
content than asteroids and more favorable orbital and impact characteristics
than Halley-type comets. In Figure
1, the amount of water delivered to Mercury from
individual impactors in one simulation is shown as a function of
(a) mass of impactor, and (b) collision probability. Asteroids are
in green, Jupiter-family comets are in red, and Halley-type comets
are in blue. Each impactor is marked by an open circle; stars depict
the objects that deliver the most water in each simulation. Note
that most of the water is supplied by a small number of objects
with high masses and average-to-high impact probabilities.

Note: This
material was cannibalized from Moses et al. (1999, Icarus137, p. 197). Further information about Mercury's polar deposits
can be found within these papers and references therein.